<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    用户名：<input id="username" />
    <script>
      document.getElementById("username").addEventListener(
        "keyup",
        (event) => {
          console.log(
            "用户" + ["不存在", "存在"][new Date().getUTCMilliseconds() % 2]
          );
        },
        false
      );
    </script>
  </body>
</html>
